This invention relates generally to a novel method and apparatus for preparing test samples for application onto a thin-layer chromatographic plate. More particularly, this invention relates to a method and apparatus for applying extremely small samples to thin-layer chromatography (TLC) plates where the solvent carrying the sample is evaporated prior to a transfer of the sample to the TLC plates.
In the past, it has been common to mix a sample together with a carrier solvent and then apply the mixture of sample and solvent to a TLC plate. The solvent would then be evaporated after application of the mixture. Devices such as syringes with very fine needles or capillary pipettes have been used to apply the sample mixture to TLC plates.
A wide variety of methods and apparatus have been available to enhance evaporation of the solvent from the mixture applied to the TLC plate or sheet. For example, vacuum sources or heaters may be placed in the vicinity of the sample. In any event, in the prior art, evaporation took place on the TLC plate leaving the sample. Typically, the evaporation took place in stages leaving small concentric rings of sample material at the location where the mixture was originally applied with the result that a sample might be present over the entire area where the mixture was originally applied to the TLC plate.
In instances where a very small sample is required, for example, in high-performance thin layer chromatography (HPTLC), greater care is required in applying the sample mixture to the TLC plate. For example, it has been known to use a platinum-iridium capillary pipette countered-balanced in a delicate instrument to provide a sample having diameter in the order of 2 mm. In HPTLC it has been found that a small, highly concentrated sample provides more satisfactory results than a larger, more dispersed sample.
While prior art arrangements have exhibited at least a degree of utility in providing a suitably small sample for HPTLC, room for significant improvement remains. It is believed that extremely small samples may be applied with greater consistency in size. Moreover, with known techniques, a painstaking procedure is required for the preparation of each sample, and it is believed that the speed of application may be significantly improved to facilitate the handling of large numbers of samples, for example, in instances where HPTLC is used to determine the presence of drugs in blood serum. In such instances, large numbers of samples need to be quickly and accurately prepared, preferably simultaneously.
The prior art includes several patent disclosures containing subject matter directed to methods and apparatus for preparing samples or specimens for chromatographic analysis, for example, U.S. Pat. No. 3,266,554, to Brownrigg and U.S. Pat. No. 3,757,952 to Baitsholts, et al.
The Brownrigg patent discloses a method and apparatus whereby syringes attached to tubes are used to deposit a droplet onto absorbent paper. Air is drawn through the absorbent paper to facilitate evaporation of the solvent carrying the sample. The size of the sample is controlled by positioning the tube in relation to the absorbent paper.
The Baitsholts et al. patent discloses a template for applying samples to a chromatographic sheet. A template having apertures which function as "wells" is positioned over a TLC plate. The samples are introduced into the wells which function to prevent spreading of the samples along the chromatographic sheet while the samples dry.
With both of these prior art devices, the evaporation of the solvent takes place on the TLC plate itself, which may result in problems of the type noted above. In addition, it is believed that these prior art devices are not suitable to provide rapid application of large numbers of samples simultaneously.
The problems enumerated in the foregoing are not intended to be exhaustive but rather are among many which tend to impair the effectiveness of previously known methods and apparatus for preparing and applying samples for use in a thin layer chromatography. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that methods and apparatus appearing in the prior art have not been altogether satisfactory.